The top 3 bloodiest wars, counted by number of fatalities, were:
- 30,000,000-60,000,000 - Mongol Conquests (13th century)
Moving on down from there, we get:
- 7,000,000 - 20,000,000 Conquests of Tamerlane (1370-1405)
Vietnam
- 2,300,000-3,800,000 - Vietnam War (entire war 1945-1975)
...
If you keep going on down the list, you'll eventually get to:
618,000 - 970,000 -
American Civil War (latter figure including 350,000 from disease) (1861-1865).
[source: Wikipedia]